Electrolytic, Theory of Dielectrics. 473 



The measured value of K for quartz is 4- 6, and of M 7 5 x 10 u . 

 Hence, for quartz, 



q = 2-3xl0~". 



If K be calculated from the index of refraction (//,) of quartz 

 its value is 2*4, which makes 



q= 1-4x10-". 



(As, however, M was measured by a " slow " method, the 

 first value of q is likely to be the more correct.) 



For tourmaline K (obtained from fi) = 2' 7, M = l*3 x 10 12 . 

 Hence 



q= 2*1x10-". 



For glass K = 7 (about) for slow electrification, and 

 M=5-5xl0 u , 



q = 2-6x10"". 



These values are about double the ionic charge of oxygen, 

 so far as it is known ; and the method of calculation is at the 

 same time far more reliable than those that have hitherto 

 been discussed. There are no very uncertain assumptions in 

 it ; and the only quantity not accurately known is d, which 

 enters equally into all the methods. It is therefore satis- 

 factory that the results obtained by this particular method 

 should be so good. 



Applying now these considerations to liquids and gases, the 

 results are at first sight less satisfactory. 



Taking water as an example of a liquid dielectric, K = l*8 

 (calculated from /i) ; M* = 2 x 10 10 , 



? = l-8xl0- 12 . 

 For bisulphide of carbon K= 1*8 (measured), M= 1*6 x 10 10 , 



q = 1-6 X 10- 12 . 

 These values are small; and if we take gaseous oxygen f the 



* M is now the coefficient of volume elasticity, of course. For solids 

 it makes very little difference, theoretically or practically, whether this 

 or Young's Modulus is used. 



t In using the formula for a gas there are one or two modifications to 

 be made. The number of molecules in the protruded (fictive) layer of 

 electrified particles at either end of the gas is d/d x 3 per sq. cm., where d 

 is the thickness of the layer ( = a xl0~ 8 ) as before, and d x is the distance 

 apart of the gas-molecules, say 5-3x10-8 (=dx ^1200). Hence the 

 equation for q comes to he 



V 



(K-l)M^^ 5 



2tt 



It is worth noticing that the direct variation of (K- 1) with the density 

 of the gas (l/d t 3 ) follows from this as it should do. 



